Sulfur dioxide is a constituent of many industrial waste gas streams such as, for example, smelter gases, flue gases, off-gases from chemical manufacturing processes, ore roasting gases, and stack gases from furnaces and boilers burning sulfur-containing fuels. Contamination of the atmosphere by sulfur dioxide has been a problem for many years due to the irritating effect of sulfur dioxide on the respiratory system, its adverse effect on plant life, and its corrosive attack of metals, fabrics, and building materials.
Millions of tons of sulfur dioxide are emitted to the atmosphere in the United States annually by the conbustion of the sulfur-containing coal and oil. It has been estimated, for example, that nearly 50% of the 30-50 million tons of sulfur dioxide annually finding its way into the atmosphere from stationary sources, results from fossil fueled electrical generating plants.
The search to date for methods of abating sulfur dioxide air pollution has generally progressed along two lines. First, attempts to eliminate the problem at its source have led to the search for low sulfur fossil fuels, or for methods of desulfurizing sulfur-containing fossil fuels. A large number of coal and oil desulfurization processes are known, and research for newer methods in this field is continuing. However, these methods add to the cost of such fuels and, in any event, provide no solution for the problem of sulfur dioxide emission from chemical processing plants and the like.
A more promising hope for long range workable solutions to the problem of sulfur dioxide air pollution lies with the second general field of search, namely in the search for methods of removing sulfur dioxide from stack gases once it is formed. Such methods provide greater versatility in their attack on the problem since they concentrate on the removal of sulfur dioxide from the waste gas streams without regard to the source. These methods provide a key to the utilization of sulfur-containing fossil fuels for electrical power generation and for the cleaner operation of sulfur related chemical and metallurgical processes.
It is estimated that there are over fifty sulfur dioxide removal processes presently under investigation in the United States. Many of these processes involve wet scrubbing processes or dry chemical absorption processes for the removal of sulfur dioxide from the waste gas stream. This is the method employed, for example, in the wet lime scrubbing process which results in the production from sulfur dioxide of calcium sulfite or, if an oxidation step is employed, calcium sulfate. Dry absorption processes are exemplified by the process which employs manganese dioxide to react with sulfur dioxide in flue gas streams to produce manganese sulfate.
Wet scrubbing processes for the removal of sulfur dioxide suffer disadvantages when used in the electrical power generating industry. The high stack gas temperatures and velocities encountered in such applications present serious design and implementation problems. The gas volume produced by a 1000 megawatt boiler, for example, is of the order of 1.7-2.0 million SCFM (standard cubic feet per minute) which moves through the equipment at velocities of 35-40 miles per hour. The high temperatures of such waste gas streams also require pre-cooling before any wet scrubbing step can be employed for sulfur dioxide removal. Moreover, the solids which result from such scrubbing processes, wet or dry, present solid waste disposal problems in their own right.
Catalytic reduction processes for abating sulfur dioxide content in waste gas streams do not suffer from these drawbacks. Using catalysts to act upon the constituents of the gas stream, and operating at relatively high temperatures and flow rates, these dry processes efficiently utilize the conditions inherent in industrial waste gas streams. These processes use reducing gases such as hydrogen, hydrogen sulfide, hydrocarbons, or carbon monoxide already in the waste gas stream, or deliberately injected into the stream, to reduce the sulfur dioxide on the catalyst surface.
In the case of carbon monoxide reduction of sulfur dioxide, the reaction proceeds according to the following reaction: EQU 2 CO+SO.sub.2 =2 CO.sub.2 +1/2 S.sub.2
in the absence of a catalyst, the above reaction proceeds very slowly even at 950.degree. C. Although thermodynamic calculations give an equilibrium constant of 410 for the reaction of 1350.degree. K., going as high at 10.sup.5 at 1000.degree. K. and 10.sup.8 at 800.degree. K. Lower reaction temperature favor the reduction of sulfur dioxide to elemental sulfur, but increasingly favor the undesirable formation of carbonyl sulfide, COS. If water is present in the waste gas stream, some hydrogen sulfide may also be formed at lower temperatures by reaction with elemental sulfur.
A wide variety of catalysts have been employed for the reduction of sulfur dioxide to sulfur by various reducing gases, but to the best of the applicants' knowledge, most suffer from one or more of three major difficulties.
First, many catalysts effective in the sulfur dioxide reduction reactions are poisoned by oxygen. This presents a particular problem in the electrical power generating industry where burners are often run on lean fuel mixtures containing excess air to prevent the formation of explosive carbon dust and to more efficiently utilize fuel. As a result, the oxygen contained in the air-rich waste gas stream poisons some catalysts employed to remove sulfur dioxide.
Second, some of the catalysts employed in the reduction of sulfur dioxide to elemental sulfur also efficiently catalyze undesirable side reactions. For example, some catalysts which have been investigated catalyze the reaction between water and elemental sulfur contained in the waste gas stream to produce hydrogen sulfide.
Third, certain non-specific catalysts utilized in the reaction between sulfur dioxide and carbon monoxide efficiently catalyze the reaction leading to carbonyl sulfide.